Video: riding shotgun at Goodwood in Ken Block’s 845bhp Mustang

The penny dropped at precisely the moment a marshal asked to see our wrists. Fully helmeted, engine idling - sorry, blaring - and sat on the Goodwood Festival of Speed's hillclimb start line, Ken Block was politely asked to show his wristband.

Of course, the marshal was only doing his job: ensuring Ken had the proper accreditation to run up Lord March's driveway in a fashion most detrimental to his rear tyres, and the assembled crowd's ears.

But this is Ken Block. A man whose YouTube videos have amassed more than 300 million views. A rallycross winner, and one of the most famous racers in the world. A famous rallyist sat in a now-famous car, being asked for his wristband. This level of Britishness could only come from a country where a fictional secret agent needs a licence to kill.

TopGear.com's guide to Ken Block's Gymkhana

Still, who cares? When Ken Block asked us politely if we'd like to join a very select group of people - numbering less than ten in the world - to have a ride in his 845bhp, all-wheel-drive, modified Mustang for a run up the Goodwood hill, TG politely jumped up and down and screamed like a small child in the affirmative.

It's quite a beautiful, brutal thing, too, isn't it? "The concept came to me about doing a car that would be very unexpected for being AWD," Block tells TopGear.com. "I actually started off in my head with a 1970s Ford Maverick. A member of my family used to own one, and I thought there was a good relationship with it.

"But I'm a really big ‘60s Mustang fan, especially those early years when they were so squared off and tough looking. I really love a lot of the muscle cars that are a bit raw."

So, make and model decided upon, Ken needed to actually source a Mustang. But not a good one.

"I definitely didn't want to take a good car and do this to it," he laughs. "We actually went out of our way to find something that wasn't really nice. That's always a big concern, people think ‘oh, there's not many of these left'.

"But actually, they made a lot of these cars. In any case, this one we found was actually a bit of a beater," he says.

Ken had a very specific styling brief - the super-Stang had to marry the worlds of WRC, DTM and, of course, Hot Wheels. "It had to have the stacks coming out of the hood," he said. Stacks that boil down to a monster, Roush Yates-tuned 410 cubic-inch Ford engine that's practically a NASCAR motor. It's 6.7-litres of pure, naturally aspirated American firepower.

"One of the reasons we partnered with RTR and Vaughn Gittin Jr on this car was because Vaughn already drifts Mustangs and uses this motor," Ken explains. "He's got a lot of experience with those engines. The output is roughly 845bhp, all naturally aspirated, so it's a very linear power curve. It's actually very easy to drive, because the engine is so easy to rev out and control."

The whole car took around two years to build - from initial concept to finished running car - that was beset with difficultly from the outset. "It's not the kind of car that you can just take stuff off the shelf and bolt it in," Ken explains. The biggest challenge, in fact, was the size of that motor, and how low the car sits.

"Mechanically, getting the propshafts around the engine and up to the front, and getting the front differential sitting correctly so it drives right... that was all really difficult," he said.

He tells us the engine sits way back behind the front wheels, ‘which is a nice thing for the weight balance', with the front diff actually in front of the engine, because the block is so low.

"It's actually a mechanical marvel," he laughs.

Once built, Ken drove the car to a tiny parking lot two doors down from the Hoonigan offices in downtown Los Angeles, where he did some, um, testing. "I did some donuts in a small parking lot," he laughs, "just so I could make sure it was doing what it was supposed to be doing. The next day, we showed up in the morning, and we started shooting."

Yep, 20 minutes of light drifting the night before was all the prep Ken had before filming the epically smoky Gymkhana 7. "It's actually setup fairly soft," he says, "which is a good thing because it lets the car move around. It also makes it slightly lazy. When we saw what it looked like on video - it looked more aggressive - we just left it that way.

"It really needs stiffer springs, but we liked the way it looks."

Watch: Ken Block's Gymkhana 7

One thing we can attest to, however, strapped into Ken's passenger seat, engine blaring, is that the Hoonicorn is not soft at all. Or lazy. The noise from that V8 is hard-edged and deafening, as you'll hear from the video below. Sat on that start line - and after Ken has proved he is accredited to drive - he turns to me, and with a serious face, asks: "Are you ready?"

Thumbs up. He floors it, and Goodwood's genteel, sunny surroundings are vanquished in a wall of V8 thunder. My head hits the back of the seat. You can't judge the Hoonicorn's pace by any rational standards, because in this cabin, nothing makes any sense. It's just power, power, noise, brakes, then power. It feels like we're entering a black hole, and my body seems as though it'll be turned into spaghetti, imminently.

We get to the stretch of hill opposite the mansion, and Block decides to put on a show for the crowd, pitching the mighty ‘Stang into a number of donuts. I lose count how many. So much tyre smoke enters the cabin, I can't see which way we were pointing.

More power. Then some brakes. Then some power. And noise. It's just brilliantly feral. At the top of the hill, with the run complete, Block performs more donuts for the assembled crowd, parks it up, and then jumps out for a quick chat with NASCAR legend Richard Petty, who's just finished a run in his mega Superbird. You couldn't write a cooler scene if you tried.

"I'll have this thing for the rest of my life," Block says of the Mustang. "I'll drive it as much as I possibly can. It's the funnest car I've ever driven..."